A traditional motor vehicle brake system includes a plurality of fluid pressure actuated wheel brakes, a master cylinder, and a brake pedal linked to a piston assembly in the master cylinder. To apply the wheel brakes, an operator pushes on the brake pedal and linearly strokes the piston assembly to create a high fluid pressure in the wheel brakes through a plurality of hydraulic channels. Before the wheel brakes become applied, fluid expelled by the piston assembly must first take up compliance in the wheel brakes and in the hydraulic channels. To maximize response, the piston assembly typically has a relatively large effective area in order to rapidly expel a substantial volume of fluid at relatively low fluid pressure at the onset of pedal travel. To attain high fluid pressure to apply the wheel brakes without requiring that the operator apply an uncomfortably high pedal force, the traditional motor vehicle brake system also typically includes a booster which supplements the pedal force applied by the operator.
A brake-by-wire brake system is distinguished from traditional motor vehicle brake systems by the addition of an electro-hydraulic actuator having a piston which performs the function of the master cylinder piston assembly but which is driven through a speed reducer by an electric motor under the control of an electronic control module (ECM) on the motor vehicle. The electrohydraulic actuator piston has a relatively large effective area for rapid response. However, to attain high fluid pressure for wheel brake application with a piston having a large effective area, the electric motor must have a relatively high torque capacity. Such high capacity electric motors are relatively expensive and represent a significant fraction of the cost of a brake-by-wire brake system. An electro-hydraulic actuator according to this invention for a brake-by-wire brake system is an improvement over prior electro-hydraulic actuators having comparable performance characteristics in that the electro-hydraulic actuator according to this invention includes a more economical electric motor of reduced torque capacity.